1. Technical Field
This invention relates to structural connectors, and more particularly to holddowns for connecting perpendicular structural elements to resist tensile forces between them.
2. Description of the Prior Art
Holddowns are metal connectors typically used to tie wooden framing studs to concrete foundations. Holddowns are needed in areas where buildings are subject to significant seismic and wind loads. In such areas toenailing the studs to a wooden mudsill extending along the top of the concrete foundation is inadequate to anchor the building to the foundation, even though the mudsill itself may be securely bolted to the foundation. The toenailed studs may pull away from the mudsill during an earthquake or windstorm.
A holddown essentially comprises an upstanding back element attached to the stud, and a shorter horizontal base element attached to an upstanding anchor bolt embedded in the concrete. The back element is typically a flat metal plate attached to the stud by two or more conventional metal bolts. The base element typically has a single central hole through which extends the upper end of the embedded anchor bolt. A washer and threaded nut securely hold the base element on the anchor bolt. Substantially diagonal side elements oriented perpendicularly to both the base and back elements and attached along respective sides thereof provide additional structural strength.
Early holddowns were constructed from heavy gauge metal pieces welded together. The manual labor required to cut and weld the pieces made these early holddowns relatively expensive. An example of a welded holddown is shown in U.S. Pat. No. 4,192,118. To reduce manufacturing costs, holddowns have been developed which can be manufactured from continuous sheet metal stock using modern progressive die metal working equipment.
An example of a one-piece holddown, shown in U.S. Pat. No. 4,665,672, is formed from a single long thin strip of metal sheet stock. A middle portion of the strip is bent to form a seat or base member and side members. The ends of the strip are bent and overlapped to form two back members, one of which extends beyond the other. A lower set of back openings extends through the overlapped back members, while an upper one or more back openings extend only through the longer back member. The seat or base member is bolted through the mudsill to the foundation, and the back members are bolted to the stud.
The maximum load allowed on a holddown by the Uniform Building Code is primarily determined by the wooden stud's resistance to longitudinal splitting initiated by the bolts connecting the stud to the back member of the holddown. For this reason, the Code requires that the center of the hole for the lower stud bolt be spaced a minimum of seven bolt diameters from the end of the stud. This requirement is based on the assumption that the load on the stud from the stud bolts is evenly distributed between the bolts. The holddown of U.S. Pat. No. 4,665,672 does not, however, evenly distribute the load between the stud bolts. The upper stud bolt, which engages only the longer of the two back members, receives a smaller portion of the total load than does the lower stud bolt which engages both of the back members. This excess load on the lower stud bolt may initiate splitting of the stud at significantly lower overall loads than those predicted by the Code's seven bolt diameter requirement discussed above.